There are applications in the fiber optics field in which a low noise, broadband light source (supercontinuum) is of particular interest. For example, efforts are now being made toward spectral slicing wherein a common light source is used to generate a multitude of wavelength division multiplexed (WDM) signals. Such an application thus has the potential for replacing many lasers with a single light source. Other applications include, but are not limited to, frequency metrology, device characterization, dispersion measurements made on specialty fibers, and the determination of transmission characteristics of gratings. All of these various diagnostic tools may be greatly enhanced by the availability of such a broadband source.
In general, supercontinuum generation involves the launching of relatively high power laser light (typically, pulsed light) into an optical fiber, waveguide or other microstructure, wherein the laser pulse train undergoes significant spectral broadening due to nonlinear interactions in the fiber. Current efforts at supercontinuum generation, typically performed using light pulses having durations on the order of picoseconds (10−12 sec) in kilometer lengths of fiber, have unfortunately shown degradation of coherence in the generating process. In particular, additional noise has been found to be introduced into the system during the spectral broadening aspect of the process.
A relatively new type of germanium-doped silica fiber with low dispersion slope and a small effective area, referred to hereinafter as “highly nonlinear fiber”, or HNLF, has recently been developed. Although the nonlinear coefficients of HNLF are still smaller than those obtained with small core microstructured fibers, the coefficients are several times greater than those of standard transmission fibers, due to the small effective area of HNLF. Supercontinuum generation using an HNLF and a femtosecond fiber laser has been reported from various sources. In particular, U.S. Pat. No. 6,775,447 issued to J. W. Nicholson et al. on Aug. 10, 2004 discloses an HNLF supercontinuum source formed from a number of separate sections of HNLF fiber that have been fused together, each having a different dispersion value at the light source wavelength and an effective area between five and fifteen square microns.
Supercontinuum light of wavelengths spanning more than one octave have been generated in microstructured and tapered optical fibers by launching light pulses having durations on the order of femtoseconds (10−15 sec) into the ends of such microstructured or tapered fibers. The extreme spectra thus produced are useful, for example, in measuring and stabilizing pulse-to-pulse carrier envelope phase, as well as in high-precision optical frequency combs. Efforts at modeling the continuum in microstructured fibers based on a modified nonlinear Schrodinger equation have been aimed at understanding the fundamental processes involved in the spectrum generation, and show that coherence is better maintained as the launched pulses are shortened in duration from the order of picoseconds to femtoseconds.
In the provision of optical frequency combs from supercontinuum sources, there are a number of applications that require a significant degree of stability for the individual frequency components that make up the comb. For example, stable combs can be used as stable oscillators and frequency comparators, as well as provide the basis for ultra-precise optical clocks. The stabilization process requires generating various RF beat notes by beating the comb against part of itself and/or against other light sources. The signal-to-noise ratio (SNR) of these beat notes depends on the power and noise properties of the continuum. For example, when a continuum is generated with picoseconds pulses in kilometer lengths of fiber, there is substantially more phase noise in the continuum than when the continuum is generated in very short lengths of fiber (i.e., less than one meter), and with femtosecond pulses. In the past, the noise of beat notes has been improved by, for example, reducing the laser noise, adjusting the fiber's dispersion and nonlinearity, or improving the RF electronics used to measure and stabilize the beat notes.
Thus, improving the SNR in a comb stabilization arrangement is considered a significant pursuit, since it will have a direct, immediate impact on the stability and robustness of stabilized optical fiber frequency combs.